The .577-450 improved looks like someone ran a .577 Snider chambering reamer into the M-H chamber. I, too, have used Cream of Wheat, both mixed with BP and straight, with absolutely no problems. And I have probably put more rounds through my Sniders than most people have. I take most of the Snider info on the net with a grain of salt, as most of it seems to be based on experience with the Nepalese pseudo-Sniders. I have seen a few of the IMA Martini's and a Snider, and I don't think they are good for much more than wall hangers.
I still disagree totally with this idea of using .600 round balls. If anything is going to raise pressures, I would be worried about that more than cream of wheat. In the black powder era, even groove diameter bullets were rarely used. A bullet about land diameter was normally used, because the explosion of the BP would bump up the slug to fill the grooves. Not until smokeless came along did groove diameter bullets become common, after folks discovered that the bullets they used with black powder gave terrible accuracy with smokeless.
There has to be a pretty big pressure spike as a .600 bullet enters the bore, as there is not really a throat in the Snider barrel. Just a really sharp cone in front of the chamber. Not to mention, that sharp throat is often rather rough. You are far better off for safety's sake using a hollow base bullet of about .580 to .585 tops. My .590 bullets were too big for three of the Sniders I have owned, never tried it in the others. I shelved that mould years ago and have not used it since.
In my Martini Henry I use the same bullets as I use in my .45-70, the Lee .459 hollow base meant for the Springfield. I have examined recovered bullets, and they fill the rifling completely. Accuracy is okay, but could be better. Maybe a larger bullet would be better, but my point is, using black powder will bump up even a considerably undersize bullet to fill the bore. No need to force oversize slugs down a barrel that is, in many cases, 150+ year old iron.